Hitherto, with respect to a pneumatic tubeless tire, a low air permeable rubber such as butyl rubber has been employed for an innerliner to maintain air pressure of the tire. FIG. 4 is a cross-sectional view of around a topping rubber layer for casecords of a conventional typical pneumatic tire. In FIG. 4, numeral 1 represents a topping rubber layer wherein casecords 4 are arranged usually in a line. On the portion inside from the casecord 4, there is an innerliner 9 comprising butyl rubber or the like for maintaining air pressure of the tire, and on the portion outside from the the casecord 4, there is provided a sidewall layer 10. The innerliner occupies about 10% by weight of tire members constituting the tire and prevents the weight of the tire from decreasing.
On the other hand, in view of necessity for low fuel consumption, a lightweight tire is required and simplicity in the tire producing processes is also required. And, it is proposed that the topping rubber layer 1 is made of a rubber composition obtained by blending low air permeable butyl rubber and brominated isobutyl/p-methylstyrene copolymer and to leave out the innerliner 9 (refer to JP-A-55665/1994, JP-A-156007/1994 and JP-A-157648/1996).
Since a highly saturated rubber such as butyl rubber or the like is inferior in adhesion property to a diene rubber which is highly unsaturated rubber, adhesion between the topping rubber layer and other tire members such as the sidewall which contains a diene rubber as a main component is inferior and adhesion to casecords is also inferior, so that the tire to be obtained has a problem in its durability.
For example, in JP-A-157648/1996, there is disclosed, for balancing adhesion properties to casecords and low air permeability, that brominated isomonoolefin/p-alkylstyrene copolymer as a highly saturated rubber is employed for a rubber composition constituting the topping rubber layer.
To obtain such an effect, however, a blending amount of the above-mentioned copolymer must be not less than 50 parts by weight in practice and, therefore, there is a problem that adhesion to other tire members containing a diene rubber as a main component is inferior.
On the other hand, from the viewpoint of further improving durability of tire, for example, in JP-A-113007/1996 and JP-A-157648/1996, there is disclosed that a thickness of a carcass is adjusted to the specific thickness for improving durability of the rolled-up end portion around a bead portion. When the thickness of carcass itself is increased, however, the weight of the whole tire is also increased as a result.
Further, in conventional tires, since an amine antioxidant is used in general, there is a problem that resistance to flexing fatigue of the rubber composition is lowered with lapse of time.
As described above, any lightweight pneumatic tubeless tire without an innerliner having a topping rubber layer, which has a low air permeability and sufficient adhesion to both of the other tire members containing a diene rubber as a main component and casecords, has not been obtained.
An object of the present invention is to provide a lightweight pneumatic tubeless tire without an innerliner in which adhesion properties of the topping rubber layer to casecords and other tire members containing a diene rubber as a main component and durability are both excellent.
Another object of the present invention is to provide a pneumatic tire without an innerliner having a particularly excellent durability around its bead portion.